Peripheral vascular resistance is controlled in large part by reflex mechanisms involving baroreceptors, integrative areas in the medulla and sympathetic innervation to the vasculature. Reflex vasodilatation appears to be produced by two separate and simultaneously occurring vasomotor mechanisms, one related to inhibition of neurogenic vasoconstrictor activity and the other related to the release of a mediator with vasodilator properties. The objectives of this study are to investigate various properties of the reflex vasodilator system. Attention will be directed toward evaluation of the role of medullary vasodilator neurons, acquisition of evidence for liberation of histamine from sympathetic nerves during reflex vasodilatation, study of the function of the reflex vasodilator in experimental hypertension and investigation of interactions between afferent pathways of the vasodilator reflex arc.